Fuel cell power plants are well known and are commonly used to produce electrical energy from hydrogen containing reducing fluid fuel and oxygen containing oxidant reactant streams to power electrical apparatus such as stationary power plants and transportation vehicles. In fuel cell power plants of the prior art, it is known that product water generated by fuel cells of the plant is often utilized to provide water for plant systems such as fuel reformers as well as to humidify gaseous reactant streams. Such product water however presents significant freeze related problems for the plant, especially during shut down and start up of the plant in sub-freezing ambient conditions.
Solutions to such freeze related problems are disclosed in U.S. Pat. No. 6,528,194 that issued on Mar. 4, 2003 to Condit et al., and in U.S. Pat. No. 6,562,503 that issued on May 13, 2003 to Grasso et al., both of which patents are entitled “Freeze Tolerant Fuel Cell Power Plant”, and both of which are owned by the owner of all rights in the present invention. Those patents disclose the use of low freezing temperature water immiscible fluids as purge fluids, during a shut down and start up of the plant to displace water from key system components.
When the fuel cell power plant disclosed in those patents is shut down for a short term shut down, displacement valves operate to control flow of the water coolant out of a fuel cell cooling coolant loop into a freeze tolerant, open tube accumulator, and to control flow of the water immiscible fluid into the coolant loop to displace the water coolant. For a long term shut down, the same procedure is undertaken to direct the water coolant into the accumulator; to direct the water immiscible fluid into the coolant loop to displace the water coolant; and, to then drain the water immiscible fluid back into the accumulator.
To start up such a power plant after a long term shut down, the water immiscible fluid is first directed to pass from the accumulator through a heater or directly through operating fuel cells of the plant and into a recycle line to flow through open tubes of the accumulator to melt the frozen water coolant. Whenever fuel cells of the plant have attained a desired operating temperature and the water coolant within the freeze tolerant accumulator has melted, flow of the water immiscible fluid out of the accumulator is terminated, and thawed water coolant is directed to flow through the coolant loop to cool the fuel cells and manage fuel cell product water. The freeze tolerant fuel cell power plant is then in a steady-state operation wherein the water coolant continues to cycle from the accumulator through the fuel cells and back to the accumulator, and the water immiscible fluid remains stored within the accumulator. The displacement or purge of the water coolant by the water immiscible fluid out of the fuel cells and coolant loop prevents mechanical damage to the plant by preventing the freezing of the water coolant during a shutdown and start up, until the water coolant is within the freeze tolerant accumulator. Also, the low freezing temperature water immiscible fluid transfers heat from the fuel cells or an external heater to melt frozen coolant water within the accumulator upon start up.
While the approach of these known solutions to freeze protection is effective, nonetheless during steady-state operation of the plant, the water immiscible fluid is not utilized, and remains inefficiently stored within the accumulator. Also, a large volume of fuel cell product water and/or water coolant is required for efficient cooling of the plant, and such a large volume of water must be melted upon power plant start up after an extended shut down in sub-freezing ambient conditions. Therefore, there is a need for a freeze tolerant fuel cell power plant that efficiently utilizes a water immiscible purge fluid and that minimizes a volume of water used in cooling the plant.